The invention relates to a magnetron operating circuit, and more particularly to a microwave energy generator which produces the desired microwave frequency output to be applied, for example, to a microwave oven, without the undesired radiation of spurious frequencies.
Generally, the source of electromagnetic energy in any microwave apparatus, such as a domestic cooking oven, is a magnetron type device, and a conventional operating circuit for the magnetron tube uses a high voltage transformer of the leakage type to provide a constant current to the magnetron, whereby the secondary winding has a rectifier to apply high voltages to the magnetron tube. In the normal operation, the magnetron tube generates microwave energy in a predetermined frequency range, but also other electromagnetic energy in a range of spurious frequencies. The output of such spurious frequencies is radiated and transmitted from the circuit components, lead wires and other structural elements of the device. This is not desired since it may jam wireless communications.
In other words, the output of the electromagnetic energy includes a major component in the fundamental frequency and a minor component in the spurious frequencies, whereby the spurious waves are in a wide frequency range covering kHz to GHz. These spurious frequencies or waves are radiated by the circuit components in the input structure of the magnetron type device. The main or fundamental wave, however, is utilized for the energization of the microwave apparatus. For this purpose the fundamental wave is transmitted by coupling means from the output portion of the magnetron device to the microwave application apparatus.
Further, the fundamental microwaves may also be radiated from the input part of the magnetron device in the form of leakage, whereby damage in a biological sense may be caused, due to its thermal effect, depending on the strength of the microwave energy. On the other hand, the spurious electromagnetic waves have an intensity level which is low relative to that of the fundamental waves, but still can jam electromagnetic communications. Therefore, these undesirable electromagnetic waves radiated into environment must be prevented or suppressed to a level below a permissible level. Hence, it is necessary to eliminate the electromagnetic waves from the input structure of the magnetron device. The electromagnetic waves from the input structure of the magnetron device are radiated directly to the free space or radiated to the environment during the transmission along the filament leads of the magnetron tube.
The conventional method for suppressing these undesired radiations provides a metal shield means for the individual circuits and uses a filter box having feed-through capacitors and ferrite core coils connected directly to the filament leads as the input structure of the magnetron tube.
In the prior art arrangement the radiation along the filament leads is absorbed by the ferrite core coils and attenuated by the feed-through capacitors, and the radiation from shielding means may be contained but for the feed-through capacitors because the attenuation characteristic of these feed-through capacitors is frequency dependent. Generally, the filter characteristic of this arrangement is not satisfactory, especially at the lower frequency band depending upon the capacitance of the feed-through capacitors. Furthermore, the feed-through capacitor has to withstand a high voltage because it is connected between the high voltage side of the transformer and ground. The power supply normally includes a half-wave rectifier in the voltage doubling circuit and a high voltage transformer of the leakage type, whereby the series circuit of the capacitor and the diode of the rectifier are connected to the secondary winding of the high voltage transformer. Also, to improve the attenuation characteristic at the lower frequency band in such an arrangement, it is necessary to increase the capacitance of the feed-through capacitor which must be of the high-voltage type. However, it is expensive to increase the capacitance of high voltage type capacitors to obtain a satisfactory attenuation characteristic.